From DE 19 96 27 35 A1 a method is made known for the monitoring of the creep behavior of rotating components of a compressor stage or turbine stage. During the process at least one test element is fastened to a component to be monitored in a region in which comparable temperatures and operating loads occur. After a predetermined running time the creep behavior of the test element is inspected in order to derive from it the creep behavior of the component to be monitored. The test element is formed as a partially tapered metal strip which in the region of the retaining slots for the turbine blades is welded on a rotor disk on the end face.
The embodiment shown therein is considered to be disadvantageous, since the metal strip can break off during operation and then lead to damage in the gas turbine.
Moreover, it is known that the components of the rotor of a gas turbine are previously inspected for defects before their assembly in order to avoid damage which can occur during the running of the gas turbine. The rotor is built from a plurality of adjoining rotor disks and a tension bolt. In addition to the thermal stresses it is especially subjected to mechanical stresses arising as a result of centrifugal force so therefore its components are inspected for defects.
The rotor disks in particular are inspected by the known material tests, such as ultrasound for example, for defects which appear as indications, which can be present after the manufacture of the rotor disks. By this, the indications point out defects, foreign material inclusions, inhomogeneities in the material structure or even cracks. The rotor disks identified as indication-free after this initial test are then used for building the rotor. Indication-free signifies that in fact no defects are present or that defects present in the components are so small that theoretically according to a fracture mechanical calculation during the running of the gas turbine no critical cracks can originate and propagate from them.
Despite the initial test of the rotor disks, these can have defects that are unidentified or underestimated in their effect, so that, for reasons of operational reliability, after a predetermined number of starts the gas turbine is opened for service purposes and the rotor inspected in a repeat test.
The rotors have to be destacked for the test, that means being stripped down into their rotor components in order to inspect for cracks the areas of the rotor disks inside the rotor which are not externally visible and therefore not inspectable.
For the check of the individual rotor disks for cracks the already known methods are used once again.
Furthermore, it is known that by means of a deterministic analysis the permissible number of starts of the gas turbine can be determined, after which a check of the rotor components for defects is to be undertaken. With this, the fracture mechanical boundary conditions and the assumed operating stresses are selected so that the permissible number of starts is conservatively planned, in other words the permissible number of starts is underestimated.
For that purpose FIG. 5 shows a number of starts-crack length graph according to the prior art.
Shown is the propagation behavior of a crack in a rotor disk. The characteristic curve 51 in this is determined according to the aforesaid analysis. With increasing numbers of starts the crack length a increases superproportionally. During operation, however, a crack should not exceed the calculated maximum permissible crack length azul.
In order to ensure the reliable running of the gas turbine a defect is assumed which theoretically initiates a crack propagation according to the characteristic curve 51. As the maximum permissible crack length azul should not be exceeded, the permissible number of starts Nzul can therefore be determined by means of the characteristic curve 51. No later than when the permissible number of starts Nzul is reached, the rotor is stripped and the rotor disks inspected for defects.
The stripping and checking of the rotor increases, however, the duration of the service inspection and so reduces the availability of the gas turbine.